Determine the salient features of a parallel programmable interface, 8255.

• 24 I/O lines in 3 8-bit port groups - A, B, C
• A, B can be 8-bit input or output ports
• C can serve as 2 4-bit input or output ports
• 3 modes of operation:

- Mode 0: A, B, C simple input or output level sensitive ports

- Mode 1: A, B input or output ports with strobe control in C

- Mode 2: A is bidirectional with control/handshake in B and C

• A, B can only change 1 byte at a time
• C has individual bit set/reset capability
• Advantage is non-dedicated circuit can change port configuration with software and no 'glue logic'
• Ports A, B, and C are used for I/O data.
• The control register is programmed to select the operation mode of the three ports A, B, and C.
• Mode0: simple I/O mode
• Any of the ports A, B, CL and CU can be programmed as input or output.
• No control of individual bits (all bits are out or all bits are in)
• Mode 1: Ports A and B can be used as input or output ports with handshaking.
• Mode2: Port A can be used as bidirectional I/O port with handshaking